The invention relates to a method for transmitting information by means of phase modulation, in particular 2PSK. 2PSK is a digital phase modulation having two different states of the signals modulated in phase. These methods are well known and the person skilled in the art knows that the demodulated signal is not unambiguous and therefore special measures have to be taken to produce unambiguity, namely a phase recovery has to be undertaken. After demodulation it is unknown, specifically, whether a bit has the value 0 or 1. In the case of 2PSK, it is known only that the demodulated signal is made up of bits having two different values. To produce unambiguity, the unmodulated high-frequency signal may, for example, be transmitted at the same time. Another approach to a solution may be to transmit a bit sequence known to the receiving end at least at the beginning of an information transmission so that it is possible, at the receiving end, to adjust the polarity of the demodulated bits so that they have precisely said bit sequence.
Motorola, LonWorks Section 4, page 6 discloses a coding, namely the Differential Manchester Code which, according to the information at the place cited, is insensitive to reversal of the polarity on the transmission path. This states nothing other than that the reliable information transmission is not impaired if the two conductors of an electrical transmission path are interchanged.
The object of the invention is to provide a method and a system in which the expenditure on phase demodulation at the receiving end is reduced. In the method according to the invention, this object is achieved in that the Differential Manchester Code is used together with a modified phase modulation transmission method, preferably 2PSK method, in which, in contrast to the prior art, a polarity recovery is not undertaken. Accordingly, a device for polarity recovery is not necessary and does not exist in the system according to the invention.
The invention is not restricted to the Differential Manchester Code being used precisely in the form in which it is defined. On the contrary, it is possible to use for the invention any code which, like the code just mentioned, always permits the differentiation of bits 0 and 1 in an unambiguous manner independently of the phase position. For this purpose, for example, the meaning of the bits 0 and 1 could be interchanged in the Differential Manchester Code; it would also be possible to use such a modified code for the invention.
In the case of the invention, it is advantageous that the receiving-end expenditure, in particular the equipment cost, is reduced compared with the standard 2PSK demodulation.
In an embodiment of the method according to the invention of the system according to the invention, a pilot tone is transmitted by means of the method according to the invention, it being advantageous that said pilot tone (which is used, as is known, to set a particular level of the signal at the receiving end) can transmit a message.